1. Field of the Invention
The present invention relates to a distillation apparatus that carries out a distillation operation widely applied to many industrial processes, and more particularly to a heat integrated distillation apparatus.
2. Description of the Related Art
Distillation separation is a unit operation widely applied to industrial processes in general, but consumes a large amount of energy. In the industrial field, therefore, studies have been conducted on an energy saving distillation systems. Such studies have brought about development of a heat integrated distillation column (hereinafter, HIDiC) as a distillation apparatus that save much energy.
As shown in FIG. 1, a basic system of the HIDiC has a structure where a rectifying section (high-pressure unit) and a stripping section (low-pressure unit) are provided such that they are separate from each other. Operation pressure of the rectifying section is set higher than that of the stripping section so that the operation temperature of the rectifying section can be higher than that of the stripping section. This enables a reduction in the amount of heat that is supplied to a reboiler because heat transfer occurs from the rectifying section to the stripping section when there is a heat-exchange surface therebetween. Heat of the rectifying section moves to the stripping section, and hence the amount of heat that is supplied at a reboiler can be reduced. As a result, high energy saving distillation apparatus can be achieved.
In order to put the concept of HIDiC to practical use, a number of distillation apparatuses having double-pipe structures, that is, double-pipe structures constituted of inner pipes forming rectifying sections and outer pipes forming stripping sections (refer to JP2004-16928A) have been proposed. These configurations are described as being capable of reducing the amounts of heat that are supplied to the reboilers and the amounts of heat that are removed at the condensers, since heat transfer occurs from the rectifying sections (inner pipes) to the stripping sections (outer pipes).
However, the heat integrated distillation apparatus having the rectifying section and the stripping section formed into the double-pipe structures as discussed in Patent Literature 1 had the following problems 1) to 6).
1) The product cannot be obtained with side-cut stream. The side-cutting means that a product is withdrawn as an intermediate distillate product, during a distillation process until an end distillate is acquired from top of column.
In the distillation apparatus described in JP2004-16928A, the tube units of the double-pipe structures are arranged to come into contact with each other. Moreover, the outer pipes and the inner pipes are equipped with the structured packing. As a result, no pipe arrangement can be formed to withdraw any intermediate distillate product from the inner pipe of each tube unit. Consequently, the structure disables side-cutting.
2) The feed stage where feed stream is provided cannot be optimized. This is because in the rectifying section and the stripping section formed into the double-pipe structures, packing heights thereof are equal, disabling free setting of the number of stages of the rectifying section and the stripping section.
3) The feed stage cannot be changed so as to meet the feed stream composition. This is because of the structure where free setting of the feeding stage position is disabled as described in 2).
4) Multi-feed stream (reception of a plurality of feed streams) cannot be dealt with. This is because of the structure where no feed stream can be supplied in the midway of the double-pipes as described in 1).
5) Maintenance of the apparatus is difficult. The tube units that use the structured packing are densely arranged to be adjacent to each other as described in 1). This disables complete access to the desired tube unit, and maintenance thereof cannot be carried out.
6) The heat exchanged rate between the rectifying section and the stripping section that uses double-pipes and in which there is no a degree of freedom in design for designing the heat transfer area, depends only on the temperature profile of the distillation column. Hence, in apparatus design, a degree of freedom in design of heat exchanged rate is small.
Q, the heat exchanged rate between the rectifying section and the stripping section, is represented by Q=U×A×ΔT, where U is an overall heat-transfer coefficient, A is a heat transfer area, and ΔT is a temperature difference between the rectifying section and the stripping section. In the HIDiC using the double-pipe structure, an inner pipe wall surface becomes a heat transfer area. This heat transfer area has a fixed value determined by a structure of the double-pipes. The overall heat-transfer coefficient also has a fixed value determined by the heat transfer structure and fluid physical properties involved in heat exchange. Thus, as can be understood from the heat exchanged rate formula, a heat exchanged rate on design specification can be changed based only on the temperature difference between the rectifying section and the stripping section, which is changed by the operating pressure of the rectifying section and the stripping section.
As the heat integrated distillation apparatus that can solve the problem as described above, the present applicant has proposed the apparatus of JP4803470B.
FIG. 2 shows an example of the distillation apparatus disclosed in JP4803470B. The distillation apparatus includes rectifying column 1, stripping column 2 located higher than rectifying column 1, first pipe 23 for communicating column top 2c of the stripping column with column bottom 1a of the rectifying column, and compressor 4 configured to compress vapor from column top 2c of the stripping column to feed the compressed vapor to column bottom 1a of the rectifying column. The distillation apparatus further includes tube-bundle-type heat exchanger 8 located at a predetermined stage of rectifying column 1, liquid withdrawal unit 2d located at a predetermined stage of stripping column 2 and configured to withdraw a part of liquid from the predetermined stage to the outside of the column, second pipe 24 for introducing the liquid from liquid withdrawal unit 2d to heat exchanger 8, and third pipe 25 for introducing fluids introduced through second pipe 24 to heat exchanger 8 and then discharged out of heat exchanger 8 to a stage directly below liquid withdrawal unit 2d. 
In the heat integrated distillation apparatus according to the constitution of the present invention, the fluids flow from stripping column 2 to heat exchanger 8 of rectifying column 1 through second pipe 24. Heat is removed from the vapor of rectifying column 1 in heat exchanger 8. Then, the heat can be transferred from rectifying column 1 to stripping column 2 through third pipe 25. The fluids flow from stripping column 2 to rectifying column 1 by gravity. The fluids in heat exchanger 8 are accordingly pushed to flow from rectifying column 1 to stripping column 2. In other words, this heat integrated distillation apparatus employs a thermo-siphon system, and hence no pressure-feeding means such as a pump is necessary for supplying the liquid from rectifying column 1 to stripping column 2 located above in a vertical direction.
With the above described apparatus configuration, which transfers heat from rectifying column 1 to stripping column 2 by using second pipe 24, third pipe 25 and heat exchanger 8, as compared with a distillation apparatus including no such heat transfer configuration, the heat exchanged rate removed from condenser 7 attached to the column top of rectifying column 1 can be reduced more, and the heat exchanged rate that is supplied to reboiler 3 attached to the column bottom of stripping column 2 can be reduced more. As a result, a distillation apparatus that is very high in energy efficiency can be provided.
Rectifying column 1 and stripping column 2 can be configured by using trayed sections or packed bed sections similar to those of a general distillation apparatus. Hence, the apparatus can deal with side cutting or multi-feed stream without the need for any improvement, and it is possible to easily perform maintenance of the apparatus. For the same reason, the number of stages of the rectifying column or the stripping column can be freely set, and a feed stage can be optimized.
A heat transfer area can be freely set, and hence the heat exchanged rate can be determined without any dependence on the temperature difference between the columns.
As described above, according to the apparatus example described in JP4803470B (FIG. 2), energy efficiency is high, side-cutting and setting of a feed stage position can be easily dealt with, and maintenance of the apparatus is easy. Further, the apparatus of the present invention has a structure in which a degree of freedom in design is high, and hence can be easily accepted by the user side.
Concerning the distillation apparatus shown in FIG. 2, the present inventors aim at further enhancement in energy efficiency, and consider that the distillation apparatus still has a room to be improved.
In other words, in the distillation apparatus shown in FIG. 2, the following method is adopted. Part or whole of the liquid in an arbitrary stage of stripping column 2 is withdrawn through pipe 24 outside the column, and is supplied to tube-bundle-type heat exchanger 8 located at an arbitrary stage of rectifying column 1, where heat exchange is performed. Thereafter, the liquid and vapor in heat exchanger 8 of rectifying column 1 pass through pipe 25 outside the column to return to directly below the above described liquid withdrawal position of stripping column 2 by the thermo-siphon effect, without being given energy from the outside by a pump or the like. Such circulation of the fluids is performed.
In such a method, a liquid head is needed at the supply side of tube-bundle-type heat exchanger 8 (pipe 24 outside the column) in order to perform circulation of the fluids by the thermo-siphon effect. In other words, as the portions extending in the vertical direction, of pipes 24 and 25 become long correspondingly to the distance (height) between liquid withdrawal position X from stripping column 2 and heat exchanger installation position Y of rectifying column 1, pressure loss through pipe 25 increases. Hence, in order to circulate the fluids by surpassing this, the liquid head based on the inlet position of heat exchanger 8 (end portion of pipe 24 connected with heat exchanger 8) also becomes large. In the tube of heat exchanger 8, however, the pressure becomes high and the boiling point increases due to the increase in the liquid head. Therefore, the temperature difference between the inside of the tube and the outside (shell) of the tube in heat exchanger 8 becomes small correspondingly to the increase of the boiling point. In order to compensate this, a necessity arises to increase the pressure of rectifying column 1, that is, to increase the temperature in rectifying column 1 by increasing the compression ratio of compressor 4. Thus, there is a room to be improved from the viewpoint of energy saving performance.
In addition, in the distillation apparatus shown in FIG. 2, such a state is brought about that the vapor rate and the liquid rate are small in the vicinity of column top 1c of rectifying column 1. In the vicinity of column bottom 1a of rectifying column 1, the vapor rate and the liquid rate are large. Thus, if the column diameter of rectifying column 1 is designed based on the vapor rate and the liquid rate in column bottom 1a of rectifying column 1, the column diameter becomes excessive in the vicinity of column top 1c. Meanwhile, such a state is brought about that in the vicinity of column top 2c of stripping column 2, the vapor rate and the liquid rate are large, and in the vicinity of column bottom 2a of stripping column 2, the vapor rate and the liquid rate are small. Therefore, as in the case of rectifying column 1, if the column diameter of stripping column 2 is designed based on the vapor rate and the liquid rate at column top 2c of stripping column 2, the column diameter becomes excessive in the vicinity of column bottom 2a. Therefore, there is also a room to be improved from the viewpoint of the structure and manufacturing cost.